The present invention relates to processes and apparatus for applying tabs to traveling webs, and more specifically to a method and apparatus for applying tabs to a traveling web of material at a transfer position when the tabs are provided to the transfer position in a path that can be skew to the travel direction of the web of material. The invention has particular applicability to the manufacture of disposable diapers.
The history of cutting and applying tabs to disposable diaper webs is now entering its fourth decade. Over the course of that time, various types of automatic manufacturing equipment have been developed which produce the desired results with a variety of materials and configurations. This equipment generally included window-knife and slip-and-cut applicators, each having their own advantages and limitations.
Window-knife applicators generally comprise the following: one or more rotating heads, each made up of a knife edge and a vacuum plate; a more or less stationary knife, which is configured with a hole (window); and a tape transfer mechanism. Typically, the rotating heads are mechanically configured to eliminate head rotation relative to the stationary knife. Each head is passed, once per cycle, across the face of the stationary window knife, through which the infeeding tape is passed. The rotating knife shears the extended length of tape against the sharp inner edge of the hole (window), after which the severed segment is held by the vacuum plate. The rotating head, with the segment of tape held in place by the vacuum plate, continues through its rotation to a point, usually 90 degrees later, where it contacts the traveling web, which is pressed against the exposed adhesive of the tape segment. This contact, usually against some backing device, effects a transfer of the tape tab from the vacuum plate to the traveling web, which then carries the tape tab downstream.
Slip-and-cut applicators are typically comprised of the following: a cylindrical rotating vacuum anvil; a rotating knife roll; and a transfer device. In typical applications, a tape web is fed at a relatively low speed along the vacuum face of the rotating anvil, which is moving at a relatively higher surface speed and upon which the tape web is allowed to “slip.” A knife-edge, mounted on the rotating knife roll, cuts a segment of tape from the tape web against the anvil face. This knife-edge is preferably moving at a surface velocity similar to that of the anvil's circumference. Once cut, the tape tab is held by vacuum drawn through holes on the anvil's face as it is carried at the anvil's speed downstream to the transfer point where the tape segment is transferred to the traveling web.
Continual improvements and competitive pressures have incrementally increased the operational speeds of disposable diaper converters. As speeds increased, the mechanical integrity and operational capabilities of the applicators had to be improved accordingly. As a further complication, the complexity of the tape tabs being attached has also increased. That is, for a given tape web design, cut tape tabs may need to be applied to a traveling web of material at a transfer position while the tape tabs are provided to the transfer position in a path that is skew to the travel direction of the web of material. Consumer product manufacturers offer tapes which are die-cut to complex profiles and which may be constructed of materials incompatible with existing applicators. For instance, a proposed tape tab may be a die-profiled elastic textile, instead of a typical straight-cut stiff-paper and plastic type used in the past. Consequently, a manufacturer may find itself with a window-knife applicator, which cannot feed a tape web with too little axial stiffness. It could also find itself with a slip-and-cut applicator, which cannot successfully apply die-cut tape segments. Furthermore, existing applicators cannot successfully apply tapes whose boundaries are fully profiled, as may be desired to eliminate sharp corners, which might irritate a baby's delicate skin. This demonstrates a clear need for an improved applicator capable of applying new tape configurations and overcoming other shortcomings of some prior art applicators.
To overcome some shortcomings, Parish et al. (U.S. Pat. No. 6,475,325), which has been assigned to the same assignee as the present application, discloses an applicator and method that allows tape tabs to be applied to a running web of material, even when the web of tape tab material is moving at a different speed than the web of material. A protuberance acting against the web of material brings the web into contact with the tape tabs and adheres the tape tabs to the web. While this invention adequately solved many of the problems of the prior art, it did not address the placement of tape tabs which are being fed in a direction that is skew to the traveling direction of the web of material.